This invention relates to graphic recording systems and especially to a system adapted for graphically recording information received over a plurality of information channels.
Electro-graphic line recording systems are frequently utilized with underwater sonar systems for recording in graphical form information relating to horizontal sea bed topography and sub-bottom structure or bathymetric profile. This is accomplished by towing a so-called "towed vehicle" containing sonar transducers behind a ship (or by mounting the transducers on the ship) while the transducers transmit sonar or acoustical signals toward the sea floor and receive reflections from the floor and from sub-floor layers. This received information is then transmitted to a graphic line recorder which records the information in the form of line traces on chart paper.
The graphic line recorders typically consist of a mechanical stylus which moves across the surface of chart paper which itself is moving in a direction generally perpendicular to the movement of the stylus. As the stylus moves across the paper, it records on the paper a line trace varying in darkness along the length of the trace to indicate the location of objects or sub-bottom layers encountered by the transmitted acoustical signals.
With the above-described prior art systems, the received information is recorded in real time as it is received from the transducers. Because of this, the distance scale factor for the tracings can be changed or varied only by changing the speed of travel of the stylus. For example, if a distance were to be represented on the chart in a narrow space, then the stylus would be moved at low speed whereas if the distance were to be represented in a wider space, then the stylus would be moved at a higher speed. Changing distance scale factors by changing stylus speed places upper limits on what scale factor (range) can be achieved because of the practical upper limits on the speed at which the stylus can be moved. Also, if different scale factors were to be used for each channel (whose information would be recorded in side by side spaces on the chart paper) then the stylus would have to move through one of the spaces corresponding to one of the channels at one speed, and then through an adjacent space corresponding to a different channel at a different speed, etc. In other words, the stylus would be controlled to change speeds as it made one pass over the chart paper. The control of such changes in speed is difficult and costly.
Another drawback of prior art recording systems also arises from the real time nature of their operation. Because the sonar information is recorded on the chart paper as it is received from the towed vehicle as already mentioned, only one transducer at a time can be operated. Thus, a side-looking transducer may be operated followed by the other side-looking transducer, in turn, followed by a sub-bottom or bathymetric transducer. The information sequentially received from the operation of these transducers is then sequentially applied to the graphic line recorder unit. Of course, because the transducers must be operated in sequence rather than simultaneously, either the sonar beams must be fairly wide in order to get complete coverage from one transmission of a transducer to the next transmission, or the towed vehicle must be moved fairly slowly. In other words, to get complete coverage of the sea floor, the towed vehicle should not be moved more than the width of a sonar beam between transmission (and receptions) by a transducer. Of course, the wider the sonar beam, the faster can the towed vehicle be moved. However, better resolution is obtained with a narrow beam and so it is preferable to use a narrower beam and, if possible, make more transmissions (and receptions). And, it would be especially desirable to be able to transmit simultaneously from a plurality of transducers in the towed vehicle rather than sequentially as is presently done with most prior art systems. It should be mentioned here that with so-called helix recorders, simultaneous transmission, at least from the side-looking transducers, is possible, but change of scale factors must still be done by varying the speed of movement of the recording element (helix drum).
A final comment may be made about prior art systems with regard to the so-called center-out side scan displays. With a recorder unit which utilizes a single stylus belt, the center-out side scan display is not possible. Rather, a "stacked" side by side display is produced and this is more difficult to read since there is a discontinuity between the two side scan tracings. Center-out side scan displays can be obtained with the previously-mentioned helix recorders (see "Electrosensitive Graphic Recording" by Fred W. Simpkins, Measurement and Data, September-October, 1969) but such recorders require "wet" chart paper which some users find undesirable.